Effect of Ultrasonic Pre-treatment on Dewaterability and Moisture Distribution in Sewage Sludge

The main objective of this work was to enhance the dewatering efficiency of sewage sludge by ultrasonic irradiation. Low-field ¹H nuclear magnetic resonance (LF-NMR) was used to deduce the moisture distribution in sewage sludge. Experiments were carried out to investigate the effects of ultrasonic frequencies (20, 25, 30, 35 and 40 kHz) and treatment time (1, 2, 3, 5 and 10 min) on water content, SCOD and moisture distribution in sewage sludge. The results showed that ultrasonic irradiation is an effective pre-treatment process for sludge dewatering. When sewage sludge was pre-treated by ultrasonic irradiation at 30 kHz for 2 min, the water content of sludge decreased from 93.03 to 86.95% after centrifugal dewatering. This means the volume of residual sludge cake after ultrasonic pre-treatment is half of the volume of raw sludge cake. In addition, the NMR analysis reveals that there are three categories of water in sewage sludge, and the moisture distributions were changed by ultrasonic irradiation. For different ultrasonic frequencies, the sample with the lowest water content contains the highest free water content. While for different pre-treatment times, the water content decreased at the first 2 min, then it gradually increased with longer ultrasonic irradiation time. The SCOD of sludge samples increased continuously with ultrasonic irradiation time extended.